In a flash memory, as the memory capacity increases, the memory cell size is becoming smaller. In a very small cell, attention is therefore being paid to a MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor) type memory obtained by changing a charge trapping layer from a floating gate type to an insulating film having a charge trapping ability.
The MONOS-type memory has a structure in which a tunnel insulating film for passing charges by controlling bias voltage, a charge trapping layer, and a block insulating film for blocking current between the charge trapping layer and a control gate electrode are sequentially stacked. Since this type of memory has structural simplification and therefore realizes shrinkage, the memory is being examined for further shrinkage as a future-generation memory.
At present, to realize a very small cell using a MONOS-type memory, an attempt to introduce a material having a higher dielectric constant (high-k material) in place of a silicon oxide film conventionally used as the block insulating film is being examined. In particular, an aluminum oxide film has a dielectric constant higher than that of a silicon oxide film and has excellent retention characteristics of trapped charge. Therefore, practical use of the aluminum oxide film as a future-generation block insulating film is being examined (see, for example, J-S. Lee, et al., SSDM (2005) 200).